[ken_yap]

You didn't say what kind of drive sda was, is it SATA? If sda is SATA and sdb is PATA, that would explain why the system reverted to sda when sdb was removed. When you have a mixed system like that, there can also be a preferred booting order where PATA overrrides SATA if installed.

When you removed sdb, it reverted to the original setup. So situation at that point is one where you have installed Linux but have no GRUB at all. So the original Windows bootloader took over.

You didn't say what the mount points of sda[567] are. Those are important. When you install GRUB, using a rescue CD, /boot must be mounted in the same relative position as it would be when running. I suspect that 5 is / and 6 is boot, but you would have to look at /etc/fstab (on sda?) to tell. Then what you have to do is:

mount /dev/sda5 /media/root
mount /dev/sda6 /media/root/boot
grub-install blah blah